Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

ABSTRACT

A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector and a positive electrode active substance layer formed upon the positive electrode current collector. The positive electrode active substance layer has a positive electrode active substance and a melamine-acid salt being a salt comprising melamine and acid.

TECHNICAL FIELD

The present invention relates to a positive electrode for a non-aqueouselectrolyte secondary battery and a non-aqueous electrolyte secondarybattery using the same.

BACKGROUND ART

In a non-aqueous electrolyte secondary battery, use of aphosphorus-containing compound has been known, for suppressing anexothermic reaction between the positive electrode active material andthe non-aqueous electrolytic solution. Patent Document 1 disclosessuppressing the exothermic reaction between the positive electrodeactive material and the non-aqueous electrolytic solution by dissolvinga phosphate ester at 15% or more by mass based on the total amount ofthe non-aqueous electrolytic solution. In addition, in Patent Document2, it is disclosed that the exothermic reaction between the positiveelectrode active material and the non-aqueous electrolytic solution issuppressed by adding polyphosphoric acid, a phosphazene derivative, orthe like in the positive electrode mixture at 6% or more by mass.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Patent No. 3131905-   Patent Document 2: Japanese Patent Laid-Open Publication No.    2010-251217

SUMMARY OF INVENTION Technical Problem

However, dissolving a large amount of the phosphate ester in thenon-aqueous electrolytic solution results in reduction of ionconductivity of the non-aqueous electrolytic solution, and a sidereaction between the phosphate ester and the negative electrode, therebydeteriorating the input-output characteristics, the charge-dischargeefficiency, and the like. In addition, the input-output characteristicsand the charge-discharge efficiency are deteriorated by adding a largeamount of polyphosphoric acid or the like in the positive electrodemixture.

It is an object of the present invention to provide a positive electrodefor a non-aqueous electrolyte secondary battery that is excellent insafety, input-output characteristics, and charge-discharge efficiency,as well as a non-aqueous electrolyte secondary battery using the same.

Solution to Problem

A positive electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention comprises a positive electrodecurrent collector, and a positive electrode active material layer formedon the positive electrode current collector, and the positive electrodeactive material layer has a positive electrode active material and amelamine-acid salt being a salt comprising melamine and acid.

In addition, a non-aqueous electrolyte secondary battery according tothe present invention comprises a positive electrode, a negativeelectrode, and a non-aqueous electrolyte, and the positive electrodecomprises a positive electrode current collector and a positiveelectrode active material layer formed on the positive electrode currentcollector, and the positive electrode active material layer has apositive electrode active material and a melamine-acid salt being a saltcomprising melamine and acid.

Advantageous Effects of Invention

The positive electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention and the non-aqueous electrolytesecondary battery using the same are excellent in safety, energydensity, and input-output characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial sectional view of an example of a positive electrodefor a non-aqueous electrolyte secondary battery according to anembodiment of the present invention.

FIG. 2 is a drawing showing exothermic behavior in DSC for an Exampleand Comparative Examples.

FIG. 3 is a drawing showing heat generation starting temperatures, peaktemperatures, and calorific values in DSC measurement results for anExample and Comparative Examples.

FIG. 4 is a drawing showing initial charge-discharge curves for anExample and Comparative Examples.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment according to the present invention will bedescribed in detail. A non-aqueous electrolyte secondary batteryaccording to the embodiment of the present invention, for example, has aconstitution in which an electrode body and a non-aqueous electrolyteare contained in an armoring body, the electrode body being formed of apositive electrode and a negative electrode wound with a separatorinterposed therebetween, or alternatively positive electrodes andnegative electrodes alternately stacked with separators interposedtherebetween. Each component of the non-aqueous electrolyte secondarybattery will be described in detail below.

[Positive Electrode]

FIG. 1 is a partially sectional view of a positive electrode 10. Thepositive electrode 10 is constituted of a positive electrode currentcollector 20 which is metal foil or the like and a positive electrodeactive material layer 22 formed on the positive electrode currentcollector 20. For the positive electrode current collector 20, there isused a foil of a metal which is stable within the potential range of thepositive electrode, a film on which a metal which is stable within thepotential range of the positive electrode is disposed as a surfacelayer, or the like. Suitably, aluminum (Al) is used as the metal stablewithin the potential range of the positive electrode. The positiveelectrode active material layer 22 is a layer which contains anelectrically conductive material 26, a binder 28, a melamine-acid salt30, and the like in addition to a positive electrode active material 24,and is obtained by mixing these materials in a suitable solvent,applying the resultant mixture to the positive electrode currentcollector 20, drying the applied material, and then rolling the driedmaterial.

For the positive electrode active material 24, there may be used atransition metal oxide containing an alkali metal element in aparticulate shape, or a transition metal oxide in which a portion of thetransition metal element contained in the above-described transitionmetal oxide has been substituted with a different kind of element.Examples of the alkali metal element include lithium (Li) and sodium(Na). Among these alkali metal elements, lithium is preferably used. Forthe transition metal element, there may be used at least one transitionmetal element selected from the group consisting of scandium (Sc),manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu),yttrium (Y), and the like. Among these transition metal elements, Mn,Co, Ni, or the like is preferably used. For the different kind ofelement, there may be used at least one different kind of elementselected from the group consisting of magnesium (Mg), aluminum (Al),lead (Pb), antimony (Sb), boron (B), and the like. Among these differentkinds of elements, Mg, Al, or the like is preferably used.

Specific examples of such the positive electrode active material 24include LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₂, LiNi_(1-y)Co_(y)O₂, (0<y<1),LiNi_(1-y-z)Co_(y)Mn_(z)O₂, (0<y+z<1), and LiFePO₄ as thelithium-containing transition metal oxides in which lithium is used asthe alkali metal element. For the positive electrode active material 24,these materials may be used singly or in combinations of two or morethereof.

The electrically conductive material 26 is a powder, a particle, or thelike having electrical conductivity and is used in order to enhance theelectron conductivity of the positive electrode active material layer22. For the electrically conductive material 26, there is used a carbonmaterial, a metal powder, an organic material, or the like havingelectrical conductivity. Specifically, examples of the electricallyconductive material 26 include acetylene black, Ketjen black, graphite,and the like as the carbon material; aluminum and the like as the metalpowder; potassium titanate, titanium oxide, and the like as metaloxides; and phenylene derivatives, and the like as the organicmaterials. These electrically conductive materials 26 may be used singlyor in combinations of two or more thereof.

The binder 28 is a polymer having a particulate shape or a networkstructure and is used in order to maintain a good contacting statebetween the positive electrode active material 24 in a particulate shapeand the electrically conductive material 26 in a powdery or particulateshape and to enhance bindability of the positive electrode activematerial 24 and the like to the surface of the positive electrodecurrent collector 20. For the binder 28, there may be used afluorine-containing polymer, an elastomeric polymer, or the like.Specifically, examples of the binder 28 include polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVdF), modified materials thereof, orthe like as fluorine-containing polymers; andethylene-propylene-isoprene copolymer, ethylene-propylene-butadienecopolymer, and the like as elastomeric polymers. The binder 28 may beused together with a thickener such as carboxymethyl cellulose (CMC),polyethylene oxide (PEO), or the like.

The melamine-acid salt 30 is a powder having flame retardance and, inthe copresence of the flammable non-aqueous electrolytic solution, has afunction as a flame retardant agent serving as a reaction inhibitor todelay the exothermic reaction of the non-aqueous electrolytic solution,thereby suppressing the calorific value. The melamine-acid salt 30 is acompound synthesized from melamine and an acid exhibiting strong flameretardance and is preferably at least one selected from melaminepolyphosphate, melamine sulfate, melamine cyanurate, melamine borate,and melamine pyrophosphate. Further, the melamine-acid salt 30 ispreferably melamine sulfate represented by the following chemicalformula (1) or melamine polyphosphate represented by the followingchemical formula (2).

[Formula 1]

[Formula 2]

It is noted that oxygen generated from the positive electrode activematerial 24 during charging is considered to oxidize the non-aqueouselectrolytic solution and this oxidation reaction is an exothermicreaction accompanied by heat generation, thereby raising the temperatureinside the battery. Accordingly, it is effective to place the flameretardant agent in the vicinity of the positive electrode activematerial 24 in order to suppress the exothermic reaction between thepositive electrode active material 24 and the non-aqueous electrolyticsolution, taking the generation of the oxygen from the positiveelectrode active material 24 into consideration.

Accordingly, the present inventors have conceived of placing themelamine-acid salt 30 composed of melamine and an acid exhibiting strongflame retardance within the positive electrode 10, thereby suppressingthe exothermic reaction between oxygen and the non-aqueous electrolyticsolution.

As described above, the melamine-acid salt 30 is preferably sparinglysoluble in the non-aqueous electrolytic solution so as to remain withinthe positive electrode active material layer 22. Solubility of thearomatic phosphate ester compound 30 in the non-aqueous electrolyticsolution was employed as the index for being sparingly soluble.

[Determination of Solubility]

Determination of the solubility was performed as follows. Ethylenecarbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate(DMC) were mixed together at a volume ratio of 3:3:4 to prepare anon-aqueous solvent. In this determination, this mixed solvent was usedas the non-aqueous electrolytic solution. Ten grams of the non-aqueouselectrolytic solution was weighed, 1 g of a melamine-acid salt 30 wasadded thereto, and the mixture was thoroughly stirred at 25° C. Then,the non-aqueous electrolytic solution was removed by filtration and theweight of the non-dissolved fraction was measured to determine thedissolved amount of the melamine-acid salt 30 in the non-aqueouselectrolytic solution. The solubility (%) of the melamine-acid salt 30in the non-aqueous electrolytic solution was calculated by dividing thedissolved amount of the melamine-acid salt 30 (g) by the weight (g) ofthe non-aqueous electrolytic solution and multiplying the resultantquotient by 100.

The solubility of the melamine-acid salt 30 in the non-aqueouselectrolytic solution is preferably 0.5% or less. There is no particularlower limit, and the solubility of 0%; i.e., being insoluble, is morepreferable.

Since the melamine-acid salt 30 can remain and be scattered within thepositive electrode active material layer 22 as described above, theparticle diameter of the melamine-acid salt 30 is preferably smallerthan that of the positive electrode active material 24. In addition, theadding quantity of the melamine-acid salt 30 may be less than that inthe case of using a flame retardant agent soluble in the non-aqueouselectrolytic solution.

In addition, the melamine-acid salt 30 exerts an excellent flameretardant effect in a smaller amount even compared to the flameretardant agent conventionally added within the positive electrode 10.Accordingly, the optimal adding quantity thereof can be calculated basedon the volume energy density in the battery characteristics, and ispreferably 1% or more by mass and 3% or less by mass based on the totalamount of the positive electrode active material layer 22. In addition,the adding quantity thereof is more preferably 1% by mass based on thetotal amount of the positive electrode active material layer 22.

[Negative Electrode]

For the negative electrode, any material which has been conventionallyused for the negative electrode in the non-aqueous electrolyte secondarybattery may be used without particular limitation. Such the negativeelectrode may be obtained by, for example, mixing a negative electrodeactive material and a binder in water or a suitable solvent, applyingthe resultant mixture to a negative electrode current collector, dryingthe applied material, and rolling the dried material.

For the negative electrode active material, any material capable ofoccluding and releasing alkali metal ions may be used without particularlimitation. For such a negative electrode active material, there may beused, for example, carbon materials, metals, alloys, metal oxides, metalnitrides, and carbon and silicon pre-occluding alkali metal. The carbonmaterials include natural graphite, artificial graphite, pitch-basedcarbon fiber, and the like. Specific examples of the metals or alloysinclude lithium (Li), silicon (Si), tin (Sn), germanium (Ge), indium(In), gallium (Ga), lithium alloys, silicon alloys, and tin alloys. Forthe negative electrode active material, these materials may be usedsingly or in combination of two or more thereof.

For the binder, a fluorine-containing polymer, an elastomeric polymer,or the like may be used similar to the case of the positive electrode10, but there is preferably used styrene-butadiene copolymer (SBR),being an elastomeric polymer, a modified material thereof, or the like.The binder may be used together with a thickener such as carboxymethylcellulose (CMC).

For the negative electrode current collector, there is used a foil of ametal which does not form an alloy with lithium within the potentialrange of the negative electrode, or a film disposed with a metal whichdoes not form an alloy with lithium within the potential range of thenegative electrode as a surface layer, or the like. For the metal whichdoes not form an alloy with lithium within the potential range of thenegative electrode, it is suitable to use copper, which is low in cost,easily processed, and good in electron conductivity.

[Non-Aqueous Electrolyte]

The non-aqueous electrolyte contains a non-aqueous solvent, and anelectrolyte salt dissolved in the non-aqueous solvent. The non-aqueouselectrolyte is not limited to a non-aqueous electrolytic solution, beinga liquid electrolyte, but may be a solid electrolyte.

For the non-aqueous solvent, there may be used cyclic carbonates,open-chain carbonates, nitriles, amides, and the like. For the cycliccarbonates, there may be used cyclic carbonates, cyclic carboxylic acidesters, cyclic ethers, and the like. For the open-chain carbonates,there may be used open-chain esters, open-chain ethers, and the like.More specifically, there may be used ethylene carbonate (EC) and thelike for the cyclic carbonates, γ-butylolactone (γ-GBL) and the like forthe cyclic carboxylic acid esters, and ethylmethyl carbonate (EMC),dimethyl carbonate (DMC), and the like for the open-chain esters. Inaddition, there may be used halogen-substituted substances which areformed by substituting a hydrogen atom of these respective non-aqueoussolvents with a halogen atom such as a fluorine atom. Among others, itis preferred to mix EC as a cyclic carbonate which is a solvent with ahigh dielectric constant and EMC and DMC as open-chain carbonates whichare solvents with a low viscosity, and use the mixture.

For the electrolyte salt, alkali metal salts may be used and lithiumsalts are more preferable. For the lithium salts, there may be usedLiPF₆, LiBF₄, LiClO₄, and the like which have been generally used as thesupporting electrolyte in conventional non-aqueous electrolyte secondarybatteries. These lithium salts may be used singly or in combination oftwo or more thereof.

In addition, the non-aqueous electrolyte may contain an additive usedfor the purpose of forming a good coating on the positive electrode orthe negative electrode or the like. For the additive, there may be usedvinylene carbonate (VC), ethylene sulfite (ES), cyclohexylbenzene (CHB),modified substances thereof, and the like. These additives may be usedsingly or in combination of two or more thereof. The fraction of thenon-aqueous electrolyte for the additive accounts is not particularlylimited, but is suitably approximately 0.05 to 10% by mass based on thetotal amount of the non-aqueous electrolyte.

[Separator]

For the separator, there is used a porous film having ion permeabilityand insulating properties disposed between the positive electrode andthe negative electrode. The porous film may include microporous thinfilms, woven fabric, non-woven fabric, and the like. The material usedfor the separator is preferably a polyolefin, more specificallypolyethylene, polypropylene, or the like.

EXAMPLES

Hereinafter, the present invention will be more specifically illustratedin detail, referring to Example and Comparative Examples, but thepresent invention is not intended to be limited to Example below. In thefollowing examples, non-aqueous electrolyte secondary batteries used inExample 1 and Comparative Examples 1 to 3 were manufactured in order toevaluate the effects of the flame retardant agents. The specificprocedures for manufacturing the non-aqueous electrolyte secondarybatteries are as follows.

Example 1 Preparation of Positive Electrode

A lithium-containing transition metal oxide represented by the generalformula LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ was used for a positive electrodeactive material. A positive electrode was prepared as follows. First,the positive electrode active material 24 represented byLiNi_(0.5)CO_(0.2)Mn_(0.3)O₂, acetylene black serving as theelectrically conductive material 26, and polyvinylydene fluoride powderserving as the binder 28 were mixed together so that the respectivecontents were 92% by mass, 5% by mass, and 3% by mass, to give amixture. The melamine sulfate serving as a melamine-acid salt of a flameretardant agent was mixed with the mixture at 1% by mass based on themixture, and the resultant mixture was further mixed with anN-methyl-2-pyrrolidone (NMP) solution to a prepare slurry. This slurrywas applied to both surfaces of the positive electrode current collector20 made of aluminum having a thickness of 15 μm by the doctor blademethod to form the positive electrode active material layers 22. Then,the layers were compressed using a compression roller to prepare apositive electrode.

[Preparation of Negative Electrode]

For the negative electrode active material, three kinds of materials:natural graphite, artificial graphite, and artificial graphitesurface-coated with amorphous carbon, were prepared, and a blend thereofwas used. The negative electrode was prepared as follows. First, thenegative electrode active material, styrene-butadiene copolymer (SBR)serving as a binder, and carboxymethyl cellulose (CMC) serving as athickener were mixed together so that the respective contents were 98%by mass, 1% by mass, and 1% by mass to give a mixture, the mixture wasmixed with water to prepare a slurry, and then this slurry was appliedto both surfaces of a negative electrode current collector made ofcopper having a thickness of 10 μm by the doctor blade method to formnegative electrode active material layers. Then, the layers werecompressed using a compression roller to a predetermined density toprepare a negative electrode.

[Preparation of Non-Aqueous Electrolyte]

LiPF₆ was dissolved as the electrolyte salt at a concentration of 1.0mol/L in a non-aqueous solvent which had been prepared by mixingethylene carbonate (EC), ethylmethyl carbonate (EMC), and dimethylcarbonate (DMC) at a volume ratio of 3:3:4 to prepare a non-aqueouselectrolytic solution which was a liquid non-aqueous electrolyte, andthe solution was used to manufacture the battery.

[Manufacture of Cylindrical Non-Aqueous Electrolyte Secondary Battery]

Further, the positive electrode, the negative electrode, and thenon-aqueous electrolytic solution thus prepared were used to manufacturea cylindrical non-aqueous electrolyte secondary battery (hereinafter,referred to as a cylindrical battery) by the following procedures. Thatis, the positive electrode 10 prepared as described above was shaped ina size of short sides of 55 mm and long sides of 600 mm, the negativeelectrode was shaped in a size of short sides of 57 mm and long sides of620 mm, and then the positive electrode 10 and the negative electrodewere wound with a separator interposed therebetween to prepare a woundelectrode body. Subsequently, the wound electrode body was disposed withinsulation plates on the top and bottom, and was accommodated in acylindrical battery outer can made of steel having a diameter of 18 mmand a height of 65 mm, with the wound electrode body also serving as anegative electrode terminal. Then, the collector tab of the negativeelectrode was welded to the inner bottom part of the battery outer canand the collector tab of the positive electrode 10 was welded to thebottom plate part of the current interruption sealing body incorporatedwith a safety device. The non-aqueous electrolytic solution was suppliedfrom the opening of the battery outer can, and then the battery outercan was sealed with the current interruption sealing body provided witha safety valve and a current interruption device to give a cylindricalbattery. It is noted that in the cylindrical battery setup was done soas to attain negative electrode capacity/positive electrodecapacity=1.1.

[Manufacture of Coin-Type Non-Aqueous Electrolyte Secondary Battery]

The positive electrode and the non-aqueous electrolytic solutionprepared as described above were used to manufacture a coin-typenon-aqueous electrolyte secondary battery (hereinafter, referred to as acoin-type battery) by the following procedures. However, the positiveelectrode was that formed by applying the slurry on a single surface ofthe positive electrode current collector, and lithium foil was used forthe negative electrode. The positive electrode 10 prepared as describedabove was punched into a round shape having a diameter of 17 mm, and thenegative electrode was punched into a round shape having a diameter of19 mm. Subsequently, the negative electrode was crimped to the inside ofthe bottom part of a coin-type battery armoring body made of steel whichhas a diameter of 20 mm and a height of 5 mm and was composed of a lidpart and a bottom part, and upon the negative electrode, the separator,the positive electrode 10, there were disposed and contained a circularbacking plate made of steel, and a plate spring, in this order. Thenon-aqueous electrolytic solution was supplied into the bottom part ofthe battery armoring body, the bottom part was covered with the lidpart, and then the battery armoring body was caulked to be sealed togive a coin-type battery.

Example 2

A cylindrical battery and a coin-type battery for use in Example 2 weremanufactured in the same manners as those for Example 1, except thatmelamine sulfate serving as the flame retardant agent was replaced bymelamine polyphosphate.

Comparative Example 1

A cylindrical battery and a coin-type battery for use in ComparativeExample 1 were manufactured in the same manners as those for Example 1,except for adding no melamine sulfate serving as the flame retardantagent.

Comparative Example 2

A cylindrical battery and a coin-type battery for use in ComparativeExample 2 were manufactured in the same manners as those for Example 1,except that the melamine sulfate was replaced by trimethyl phosphate(TMP) represented by the chemical formula (CH₃O)₃PO as the flameretardant agent and the non-aqueous electrolytic solution used was thatin which the trimethyl phosphate had been dissolved at 10% by mass basedon the total amount of the non-aqueous electrolytic solution. It isnoted that all of the trimethyl phosphate used was dissolved in thenon-aqueous electrolytic solution, and therefore solubility wasindicated as an arbitrary amount.

[Differential Scanning Calorimetry]

Thermal analysis of the positive electrode active material 24 in a fullcharge state in the copresence of the non-aqueous electrolytic solutionwas performed by differential scanning calorimetry (DSC) for the purposeof comprehending the flame retardant effect of the flame retardantagents. The analytical procedures are as follows. Each of coin-typebatteries of Example 1 and Comparative Examples 1 and 2 was charged at aconstant current of 0.3 mA at 25° C. until the cell voltage became 4.3V. Subsequently, each coin-type battery was disassembled, the positiveelectrode was taken out from the battery armoring body, washed with thenon-aqueous solvent to remove the non-aqueous electrolytic solution, andthen 1 mg of the positive electrode active material layer was scraped,and enclosed in a pressure-resistant airtight enclosure together with 1μL of the non-aqueous electrolytic solution to be provided as ameasurement sample. For each measurement sample, the temperature wasraised from 25° C. to 550° C. at a temperature rising rate of 10° C./minusing a DSC and the initial exothermic peak temperature and thecalorific value were determined.

Table 1 shows the summarized heat generation peak temperatures andcalorific values in Examples 1 and 2 and Comparative Examples 1 and 2.

TABLE 1 Flame retardant agent Adding Starting Peak Calorific quantitySolubility temperature temperature value Name of compound (% by mass)(%) (° C.) (° C.) (J/g) Example 1 melamine sulfate 1 <0.1 287 308 1023Example 2 melamine polyphosphate 1 <0.1 287 304 1010 Comparative Notadded 0 — 284 298 1126 Example 1 Comparative Trimethyl phosphate  10*¹Arbitrary 303 310 1041 Example 2 amount *¹The added quantity in thenon-aqueous electrolytic solution is indicated, since trimethylphosphate is soluble therein.

FIG. 2 shows the exothermic behavior in DSC for an Examples 1 and 2 andComparative Examples 1 and 2. FIG. 3 shows the summarized heatgeneration starting temperatures, heat generation peak temperatures, andcalorific values based on the results of the DSC measurements.

As shown in FIG. 3, Examples 1 and 2 resulted in a higher heatgeneration starting temperature and heat generation peak temperature anda smaller calorific value compared to those in Comparative Example 1.That is, the melamine-acid salt 30, when present within the positiveelectrode 10, was able to raise the heat generation starting temperaturein the exothermic reaction between the positive electrode activematerial 24 and the non-aqueous electrolytic solution, shift the heatgeneration peak to the higher temperature side even when the heatgeneration started, and make the calorific value smaller. In this way,the melamine-acid salt 30, when present within the positive electrode10, exerts a flame retardant effect.

In addition, Examples 1 and 2 resulted in smaller calorific values eventhough the heat generation starting temperatures and the heat generationpeak temperatures appeared at the lower temperature side, compared tothose in Comparative Example 2. That is, the calorific value was reducedby arranging the melamine-acid salt 30 within the positive electrode 10.In this way, the melamine-acid salt 30, when present within the positiveelectrode 10, is more excellent in the flame retardant effect comparedto trimethyl phosphate, which is soluble in the non-aqueous electrolyticsolution.

[Evaluation of Initial Charge-Discharge Characteristics]

Then, the initial charge-discharge characteristics were evaluated forthe purpose of comprehending the charge-discharge characteristics in thecase of adding the flame retardant agent. The evaluation method is asfollows. Each of the cylindrical batteries in Example 1 and ComparativeExamples 1 and 2 was charged at a constant current of 250 mA at 25° C.until the cell voltage became 4.2 V, and charged at a constant voltageafter the cell voltage reached 4.2 V. After the charge current valuereached 50 mA, each battery was discharged at a constant current of 250mA until the cell voltage became 2.5 V. The charge-discharge efficiencywas determined by dividing the discharge capacity by the charge capacityand multiplying the resultant quotient by 100.

Table 2 summarizes the charge capacities, the discharge capacities, andthe charge-discharge efficiencies in Example 1 and Comparative Examples1 and 2. In addition, FIG. 4 shows the charge curves and the dischargecurves in Example 1 and Comparative Examples 1 and 2.

TABLE 2 Flame retardant agent Charge- Adding Charge Discharge dischargequantity capacity capacity efficiency Name of compound (% by mass)Solubility (%) (mAh) (mAh) (%) Example 1 melamine sulfate 1 <0.1 14251407 98.7 Comparative Not added 0 — 1428 1412 98.9 Example 1 ComparativeTrimethyl phosphate  10*¹ Arbitrary 1431 1395 97.5 Example 2 amount*¹The added quantity in the non-aqueous electrolytic solution isindicated, since trimethyl phosphate is soluble therein.

As shown in Table 2 and FIG. 4, the charge capacity, the dischargecapacity, and the charge-discharge efficiency obtained in Example 1 arealmost the same as those obtained in Comparative Example 1. In this way,the melamine-acid salt 30, when present within the positive electrode 10in a suitable adding quantity in the capacity design of the battery,gave input-output characteristics and charge-discharge efficiencycomparable to those in the case without addition thereof. It is notedthat the above-described input-output characteristics refer to thecharge capacity and the discharge capacity.

In addition, in Example 1 more excellent charge-discharge efficiency wasobtained compared to that in Comparative Example 2. That is, it isconsidered that when trimethyl phosphate, which is a flame retardantagent soluble in the non-aqueous electrolytic solution, is added in thenon-aqueous electrolytic solution as in Comparative Example 2, the flameretardant agent is to be present throughout the inside of the battery,thereby reducing the ion conductivity of the non-aqueous electrolyticsolution and deteriorating the input-output characteristics and thecharge-discharge efficiency due to causing the side reaction with thenegative electrode. On the other hand, it is speculated that themelamine sulfate used in Example 1 can remain within the positiveelectrode 10 as is because of being sparingly soluble in the non-aqueouselectrolytic solution when added to the positive electrode activematerial layer 22, and therefore suppresses reduction of the ionconductivity of the non-aqueous electrolytic solution and the sidereaction in the negative electrode while not causing reduction ofcharge-discharge efficiency, thereby being able to give excellentinput-output characteristics and charge-discharge efficiency.

As described above, a flame retardant effect is exerted by using themelamine-acid salt 30. In addition, since the melamine-acid salt 30 issparingly soluble in the non-aqueous electrolytic solution, thereduction of ion conductivity of the non-aqueous electrolytic solutionand the side reaction with the negative electrode are suppressed.Further, since the melamine-acid salt 30 can exert the flame retardanteffect by addition of a smaller amount compared to the flame retardantagent added within the positive electrode 10, which has beenconventionally considered necessary to be added at 6% or more by mass,the positive electrode 10 containing the melamine-acid salt 30 isexcellent in energy density.

In this way, the positive electrode for a non-aqueous electrolytesecondary battery comprising the melamine-acid salt 30 and thenon-aqueous electrolyte secondary battery comprising the positiveelectrode for a non-aqueous electrolyte secondary battery are excellentin safety, input-output characteristics, and charge-dischargeefficiency.

REFERENCE SIGNS LIST

-   -   10 positive electrode, 20 positive electrode current collector,        22 positive electrode active material layer, 24 positive        electrode active material, 26 electrically conductive material,        28 binder, 30 melamine-acid salt.

The invention claimed is:
 1. A positive electrode for use in anon-aqueous electrolyte secondary battery, the positive electrodecomprising a positive electrode current collector and a positiveelectrode active material layer formed on the positive electrode currentcollector, the positive electrode active material layer having apositive electrode active material and a melamine-acid salt being a saltcomprising melamine and acid, the melamine-acid salt having a sizesmaller than the positive electrode active material, wherein themelamine-acid salt is melamine polyphosphate having a solubility of 0.5%or less in a non-aqueous electrolytic solution serving as a liquidnon-aqueous electrolyte, and represented by chemical formula (2):

and wherein content of the melamine-acid salt is 3% or less by massbased on the positive electrode active material layer.
 2. The positiveelectrode for a non-aqueous electrolyte secondary battery according toclaim 1, wherein the melamine-acid salt is contained at 1% or more bymass based on the positive electrode active material layer.
 3. Thepositive electrode for a non-aqueous electrolyte secondary batteryaccording to claim 1, wherein the positive electrode active materiallayer comprises an electrically conductive material and a binder.
 4. Anon-aqueous electrolyte secondary battery comprising a positiveelectrode, a negative electrode, and a non-aqueous electrolyte, thepositive electrode comprising a positive electrode current collector anda positive electrode active material layer formed on the positiveelectrode current collector, the positive electrode active materiallayer having a positive electrode active material and a melamine-acidsalt being a salt comprising melamine and acid, the melamine-acid salthaving a size smaller than the positive electrode active material,wherein the melamine-acid salt is melamine polyphosphate having asolubility of 0.5% or less in a non-aqueous electrolytic solutionserving as a liquid non-aqueous electrolyte, and represented by chemicalformula (2):

and wherein content of the melamine-acid salt is 3% or less by massbased on the positive electrode active material layer.
 5. The positiveelectrode for a non-aqueous electrolyte secondary battery according toclaim 1, wherein a particle of the melamine polyphosphate is presentclose to the positive electrode active material.
 6. The positiveelectrode for a non-aqueous electrolyte secondary battery according toclaim 1, wherein a particle of the melamine polyphosphate is in contactwith the positive electrode active material.
 7. The positive electrodefor a non-aqueous electrolyte secondary battery according to claim 1,wherein a plurality of particles of the melamine polyphosphate are incontact with a surface of the positive electrode active material.